1. Field of the Invention
The present invention relates to a method for producing a glass preform for use in the fabrication of an optical fiber.
2. Description of the Related Art
One of the methods for producing a glass preform for use in the fabrication of an optical fiber comprises synthesizing glass soot (fine particles of glass) from a glass-forming raw material by flame hydrolysis, depositing the glass soot on a periphery of a rotating starting glass rod (soot depositing to grow a porous glass body in a direction of an axis of the glass rod to form a composite of the glass rod and the deposited porous glass body and heating and sintering the composite in an electric furnace to consolidate the porous glass body, whereby a transparent glass preform is produced. In the sintering step, it is important to obtain a completely transparent glass preform having no residual bubbles or other defects.
In the sintering step, the fine particles of glass in the porous glass body are fused together so that each void in the glass body is gradually isolated. To obtain the completely transparent glass preform by shrinking and vanishing the isolated bubble, the sintering atmosphere is one of the important factors. When the porous glass body is sintered in an atmosphere of a gas which has a large solubility in glass, the transparent glass is easily obtained. It is known that, in helium, it is easy to obtain the transparent glass having no residual bubble, while in argon or nitrogen, bubbles remain in the sintered glass. Therefore, in general, the porous glass body is sintered in the helium atmosphere.
The glass preform produced by the above method is drawn to fabricate an optical fiber having an outer diameter of 100 to 200 .mu.m. In the drawing step, the glass preform is heated to a temperature of 1,900.degree. C. or higher and drawn, since the glass preform should be processed to a drawable intermediate rod having a suitable diameter for drawing. If the glass preform has defects such as minute crystalline particles or bubbles at an interface between the starting glass rod and the deposited part, such defects grow to generate large bubbles in the glass rod for drawing, which considerably decreases an yield of the glass rod for drawing. For example, when the crystalline particles or bubbles of 0.1 to 0.5 mm in diameter are contained, they grow to bubbles of 1 to 5 mm in diameter. When the glass rod for drawing having such the large bubbles is drawn to fabricate the optical fiber, the optical fiber is easily broken and cannot be continuously fabricated.